SUMMARY OF THE INVENTION
The present invention relates to a method of removing a photoresist film formed in a pattern on the surface of a semiconductor wafer and an apparatus for carrying out the method.
A photoresist (hereinafter referred to simply as a "resist") is an organic substance. A known organic film removing method called "ozone ashing" exposes a photoresist film (hereinafter referred to simply as a "resist film") to a hot gas containing ozone to remove the resist film by thermal decomposition. Another known organic film removing method removes a resist film by destroying the chemical bonds between the atoms of the resist forming the resist film with ultraviolet radiation, converting ozone into excited oxygen atoms, and applying the excited oxygen atoms after destroying the chemical bonds to the resist film to convert the resist forming the resist film into a volatile substance by decomposing the resist by oxidation. The foregoing known ozone ashing method is called an ultraviolet ozone ashing method or a photoashing method. Both the former ashing method and the latter ashing method have an advantage in that the element-forming region of a semiconductor wafer (hereinafter, referred to simply as a "wafer") exposed to an ashing atmosphere is scarcely damaged by charged particles during ashing because the ashing atmosphere contains few charged particles.
The temperature of the resist film must be increased to enhance the ashing rate because, at a high temperature, the heated resist promotes the chemical reaction between the resist, and ozone, and/or ultraviolet radiation. One method of increasing the temperature of the resist film heats the wafer on a wafer stage with an electric heater built in the wafer stage. Another method of increasing the temperature of the resist film irradiates the resist film with heating lamps. Since the resist film is formed integrally with the wafer, both the resist film and the wafer are heated simultaneously when increasing the temperature of the resist film by either of the foregoing methods.
If the wafer is exposed to heat of a high temperature during the ashing process, flaws are formed in the element-forming region of the wafer doped with impurities. When the wafer is subjected to a high-temperature ashing process, undesirable thermal diffusion of the impurities occurs in the element-forming region of the wafer, which entails defects in and malfunction of the elements of the semiconductor device, such as defects attributable to the dimensional change of the diffused layers of the semiconductor elements. Therefore, the temperature of the resist film during the ashing process (hereinafter, referred to as the "processing temperature") must be lower than a critical temperature above which substantial defects are formed in the elements. This critical temperature decreases with the increase of the degree of integration of the semiconductor device, because the higher the degree of integration, the greater is the possibility that the undesirable thermal diffusion of impurities causes defects in the elements. Although an upper limit processing temperature for the conventional methods has been, for example, in the range of about 250.degree. C. to about 300.degree. C., there is a tendency to lower the upper limit processing temperature to a temperature on the order of 200.degree. C. and further to a temperature on the order of 150.degree. C.
However, the decrease of the processing temperature entails significant reduction of ashing rate, because the rate of chemical reaction between the resist, ozone and/or ultraviolet radiation is retarded when the processing temperature is lowered. The reduction of the ashing rate reduces the throughput of the semiconductor device fabricating process and deteriorates the mass-productivity of the semiconductor device.
An ashing method intended to suppress the ashing rate reducing effect of the reduction of the processing temperature is disclosed in Japanese Patent Laid-open No. 4-302145. This previously proposed ashing method uses an ozone-water mixed gas prepared by mixing ozone gas and steam. It is stated in the reference that the ashing rate in an ashing process using the ozone-water mixed gas is about 1.2 times the ashing rate in an ashing process using ozone gas, provided that other ashing conditions are fixed. It is inferred that the use of the ozone-water mixed gas enhances the ashing rate because ozones decompose upon the absorption of ultraviolet light mostly of 254 nm in wavelength into radical oxygen atoms having high energy, and steam enhances the quantum efficiency in producing radical oxygen atoms.
However, the inventors of the present invention found that foreign matter remains on the surface of the wafer when the wafer is processed by this known ashing method. This problem will be described hereinafter with reference to FIG. 1.
Referring to FIG. 1, which shows a known ashing apparatus of a single wafer processing type that uses an ozone-water mixed gas prepared by mixing ozone gas and steam, a resist film of a desired pattern for use as a mask is formed over the surface of each of a plurality of wafers 14 separately stored on a wafer holder 15. The wafers 14 are fed one at a time into a processing chamber 1 defined by a processing vessel by a wafer conveying mechanism 16. The wafer 14 fed into the processing chamber 1 is mounted on a wafer stage 2 capable of supporting a single wafer at a time. The wafer stage 2 is provided internally with an electric heater 20. A temperature controller 19 regulates the electric power supplied to the electric heater 20 so that the surface of the wafer stage 2 is maintained at a desired temperature.
Although it is desirable to heat the wafer stage 2 at a comparatively high temperature to increase the ashing rate when ashing the resist film, it is also desirable that the temperature of the wafer stage 2 be comparatively low to prevent damaging the elements formed on the wafer 14. Generally, the wafer stage 2 is heated at an appropriate temperature in the range of 150.degree. C. to 250.degree. C. Since the heat capacity of the wafer stage 2 is far greater than that of the wafer 14, the wafer 14 can be heated at a temperature infinitely close to the surface temperature of the wafer stage 2. However, the temperature of the wafer 14, immediately after the wafer 14 of a temperature nearly equal to the room temperature has been mounted on the wafer stage 2, is different from the surface temperature of the wafer stage 2, and it takes some time for the temperature of the wafer 14 to rise substantially to the surface temperature of the wafer stage 2. The greater the maximum output of the electric heater 20, the shorter the time necessary for heating the wafer 14 mounted on the wafer stage 2 to a temperature equal to the surface temperature of the wafer stage 2.
A partition plate 21 formed of high-purity quartz divides the interior of the processing vessel into the processing chamber 1 and a light source chamber containing an ultraviolet light source 3. The partition plate 21 is fixedly provided with a plurality of nozzles 5 connected to a distribution tube 4. The ultraviolet light source 3 is disposed near the partition plate 21. The ultraviolet light source 3 is used to enhance the ashing rate; however, the same is not necessarily essential to ashing. A mixed gas prepared by mixing ozone gas and steam is supplied through a supply line 6 into the distribution tube 4. The ozone gas is produced by converting oxygen gas using an ozonizer 8. Oxygen gas containing about 5% ozone is supplied at a rate on the order of 10 l/min. The ozonizer 8 produces ozone gas at a fixed rate and supplies the ozone gas to the supply line 6.
An ozone-containing gas may be used instead of pure ozone gas. A water metering device 12 supplies water at a fixed rate to a steam generating device 10, and then the steam generator generates steam at a fixed rate. The steam generated by the steam generating device 10 is supplied into the supply line 6. The ozone gas and the steam are mixed at a pipe fitting 7 to produce the mixed gas. Then, the mixed gas flows through the supply line 6, the distribution tube 4 and the nozzles 5 into the processing chamber 1. Subsequently, the mixed gas flows along the surface of the wafer 14 and is discharged from the processing chamber 1 through a discharge opening 18.
It is preferable to mix ozone gas and steam at a fixed ozone/steam ratio when producing the mixed gas. If either ozone gas or steam is not metered, the steam content of the mixed gas will be excessively large or excessively small. If the steam content of the mixed gas is excessively small, the mixed gas is not sufficiently effective in increasing the ashing rate. If the steam content is excessively large, condensation of the stem contained in the mixed gas, for example, on the inner surface of the supply line 6 is liable to occur before the mixed gas reaches the surface of the wafer 14.
The mixed gas must be supplied continuously during the operation of the ashing apparatus to supply a mixed gas having a fixed steam content into the processing chamber 1. If the ozonizer 8 is not in a constant operating state or the steam generating device 10 does not generate steam at a predetermined rate, the steam content of the mixed gas will be excessively large or excessively small. Accordingly, the mixed gas must be supplied into the processing chamber 1 at least in a period between a point of time immediately before placing the wafer in the processing chamber 1 and a point of time when the resist film is removed completely.
The ashing apparatus shown in FIG. 1 and an ashing apparatus shown in FIG. 3 are suitable for single-wafer processing. When processing wafers for ashing in a single-wafer processing mode, given a repeating cycle including steps of placing a wafer in the processing chamber, removing the resist film by ashing, and taking out the wafer from the processing chamber, it is desirable to supply the processing gas continuously into the processing chamber to process wafers at a high throughput. It is preferable that the temperature of the mixed gas be high enough to prevent the condensation of steam contained in the mixed gas, for example, on the inner surface of the supply line 6 before the mixed gas reaches the surface of the wafer 14.
When a wafer having a surface temperature substantially equal to the room temperature is transferred from the outside to the inside of the processing chamber 1, the steam contained in the mixed gas condenses into water drops on the resist film pattern and the exposed portions of the surface of the wafer, because part of the moisture contained in the mixed gas containing steam and prevailing in the vicinity of the surface of the wafer is cooled locally, just like a pair of glasses become fogged when the person wearing the pair of glasses enters a heated room from the cold outside. The humidity of the processing chamber 1 is comparatively high because the mixed gas containing steam is supplied into the processing chamber 1, and the temperature of the processing chamber 1 is comparatively high because the wafer stage 2 is heated. Accordingly, it is considered that, in most cases, the condensation of steam on the surface of the wafer is unavoidable regardless of the temperature of the wafer placed in the processing chamber 1 because the ashing apparatus having the processing chamber 1 is used in a clean room.
The condensation of steam contained in the mixed gas must be discriminated from dewing. Even if dewing does not occur within the line for supplying the mixed gas, it is highly probable that condensation of steam will occur on the surface of the wafer. Generally, the condensation of steam on the surface of the wafer is more likely to occur than dewing in the mixed gas supply line, and the condensation has a more significant effect on the wafer processing than the dewing in the mixed gas supply line.
For example, the dewing can be obviated by reducing the length of the steam supply line connecting the steam generating device 10 to the processing chamber and/or heating the pipe forming the steam supply line with a heater, whereas it is difficult to prevent the condensation of steam on the surface of the wafer. It may be effective to heat the wafer beforehand in a dry atmosphere before placing the wafer in the processing chamber 1, which, however, requires an additional apparatus connected to the ashing apparatus, increases the cost of the ashing apparatus, and increases the processing steps to affect adversely the through put of the ashing apparatus.
Water drops formed on the surface of the wafer by condensation evaporate and form water marks on the surface of the wafer. Generally, water marks are whitish, translucent marks. It is considered that water marks are a layer of an oxide, such as SiO.sub.2, produced by interaction between water (H.sub.2 O) and a substance, such as Si, forming the surface of the wafer. Water marks cannot be removed by ashing because the oxide forming the water marks is an inorganic substance.
After the completion of the ashing process, the wafer is subjected to necessary semiconductor device fabricating processes including a thin film forming process in other semiconductor processing apparatuses. If water marks remain on the surface of the wafer, part of a thin film and an aluminum wiring film is formed on the water mark during such semiconductor device fabricating processes. Since the elements of a semiconductor device are formed in a minute region, water marks having a certain thickness and size are foreign matter which cause defects in the elements of the semiconductor device.
FIGS. 2(a) and 2(b) show a thin film formed on the surface of a wafer (semiconductor substrate) having a water mark. FIG. 2(a) shows a defect in the thin film caused by the water mark and FIG. 2(b) shows a wafer (semiconductor substrate) having a water mark, and a thin film formed on the wafer having the water mark and covering the water mark. There is the possibility that both the thin films shown in FIGS. 2(a) and 2(b) are unable to function properly and cause defects in the elements.
A method intended to suppress the reduction of the ashing rate attributable to the decrease of the processing temperature is proposed in Japanese Patent Laid-open No. 1-179237. This method uses a mixed gas consisting of ozone gas, oxygen, steam and hydrogen peroxide (H.sub.2 O.sub.2). The ashing rate of this method proposed in reference 2 is several times that of a method which uses ozone gas alone, provided that other conditions for ashing including the processing temperature are the same.
However, as is generally known, hydrogen peroxide is highly toxic to human bodies and hence this method entails many problems in securing the safety of operators for the ashing process. Further, since hydrogen peroxide discharges oxygen atoms, changes into water by deoxidization, and has a boiling point of 151.4.degree. C. which is higher than that of water, the mixed gas containing hydrogen peroxide is far more liable to form water marks than the mixing gas containing steam. Therefore, it is not practically desirable to use the mixed gas containing hydrogen peroxide for ozone ashing.
Accordingly, it is a first object of the present invention to provide an organic film removing method capable of removing a resist film formed in a pattern on the surface of a wafer by ashing using ozone gas or an ozone-containing gas at an ashing rate substantially equal to or higher than that at which the resist film can be removed by ashing using a steam-containing ozone gas or an ozone-containing gas under the same ashing conditions including processing temperature, and of reducing water marks more effectively than the method using a steam-containing ozone gas.
A second object of the present invention is to provide an organic film removing apparatus for carrying out the organic film removing method for achieving the first object.
To achieve the first object, the present invention provides an organic film removing method, which uses an organic film removing apparatus provided with a processing vessel defining a processing chamber in which to remove a resist film formed on the surface of a semiconductor wafer, a wafer support means disposed within the processing chamber for supporting the semiconductor wafer, and a processing gas supply means for supplying a mixed gas produced by adding an alcohol to an ozone-containing gas or ozone gas to the surface of the semiconductor wafer supported on the wafer supporting means, comprising the following steps:
supplying the mixed gas into the processing chamber for at least a period between a point of time before placing the semiconductor wafer on the wafer support means within the processing chamber and a point of time when the resist film is removed completely; PA1 supporting the semiconductor wafer on the wafer support means within the processing chamber; and PA1 heating the resist film formed on the surface of the semiconductor wafer supported on the wafer support means within the processing chamber at a temperature in a range below a temperature which will cause defects in the elements of a semiconductor device to be formed on the semiconductor wafer. PA1 a processing vessel defining a processing chamber in which to remove a resist film formed in a pattern on a semiconductor wafer; PA1 a wafer supporting means for supporting the semiconductor wafer within the processing chamber, disposed within the processing chamber; PA1 a mixed gas supply means for supplying a mixed gas produced by mixing ozone gas and an alcohol or alcohols into the processing chamber so as to be blown against a resist film formed in a pattern on the surface of the wafer supported on the wafer supporting means; and PA1 a heating means for heating the resist film formed on the surface of the wafer supported on the wafer supporting means.
Generally, the ashing rate of an ashing method which uses a mixed gas produced by adding an alcohol to ozone gas or an ozone containing gas (hereinafter referred to inclusively as "ozone gas") is equal to or higher than the ashing rate of an ashing method which uses a mixed gas consisting of steam and ozone gas. It is inferred that the ashing rate of the former ashing method is equal to or higher than that of the latter ashing method because the effect of the alcohol on the enhancement of the quantum efficiency in producing radical oxygen atoms is higher than that of steam.
Although the alcohol, as well as water, condenses at a temperature below its boiling point, generally, alcohol drops rarely form marks corresponding to the water marks on the surface of a wafer because the alcohol rarely forms an inorganic substance together with the substance, such as Si, forming the surface of the wafer. The moisture content of the mixed gas produced by adding the alcohol to ozone gas is far smaller than that of the mixed gas produced by adding steam to ozone gas and, consequently, the use of the former mixed gas is very effective in eliminating causes of water marks.
It is desirable to use an alcohol having a comparatively low boiling point because an alcohol having a comparatively low boiling point does not easily condense on the surface of the wafer and, hence, causes of development of foreign substances on the surface of the wafer can be reduced. It is preferable to use an alcohol having a boiling point that is at least lower than that of water (100.degree. C.). The use of a mixed gas containing an alcohol reduces the probability of forming foreign substances more effectively than the use of a mixed gas containing steam because the probability of formation of foreign substances when a mixed gas containing an alcohol is used is lower than that when a mixed gas containing steam is used.
Preferable alcohols are methanol, ethanol and propanol. These alcohols have boiling points lower than that of water. For example, the boiling point of ethanol is 64.5.degree. C. The mixed gas may be produced by mixing at least one of methanol, ethanol and propanol; only one of methanol, ethanol and propanol; or alcohols other than methanol, ethanol, and propanol; with ozone gas or the like. The mixed gas may additionally contain steam in a small content because the use of an alcohol or alcohols in combination with steam suppresses the formation of water marks.
The organic film removing method of the present invention is suitable particularly for removing resist films formed on wafers in a single wafer processing mode. When the organic film removing method is carried out in a single wafer processing mode, it is desirable to supply the mixed gas continuously into the processing chamber during the operation of the organic film removing apparatus because the surface treatment of the wafer is a continuous process.
A preferable quantity of a liquid alcohol or liquid alcohols at a room temperature to one liter of ozone at a room temperature under atmospheric pressure is in the range of 0.2 g to 4 g. If the quantity of liquid alcohol or liquid alcohols is less than 0.2 g, it is impossible to carry out the ashing process at an ashing rate substantially equal to or higher than that at which the ashing process is carried out by using a steam-containing mixed gas. The quantity of liquid alcohol or liquid alcohols greater than 4 g is excessively large; the surplus liquid alcohol or liquid alcohols do not contribute to ashing. Although the greater the quantity of liquid alcohol or liquid alcohols, the higher the ashing rate, the ashing rate reaches a maximum when the quantity of liquid alcohol or liquid alcohols is increased to 4 g. Therefore, 4 g is a substantial maximum quantity effective in increasing the ashing rate.
It is desirable to irradiate the surface of the wafer with ultraviolet radiation during the ashing process. The use of ultraviolet radiation having a wavelength of 185 nm and ultraviolet radiation having a wavelength of 254 nm are particularly effective for increasing the ashing rate.
Desirably, the mixed gas is produced by mixing ozone gas and an alcohol vapor produced by heating a liquid alcohol or liquid alcohols, because the latent heat of the alcohol vapor suppresses the condensation and dewing of the alcohol or alcohols.
Ozone gas and an alcohol or alcohols may be mixed either inside the processing chamber or outside the processing chamber, so long as the mixed gas is blown against the surface of the wafer.
The mixed gas may also contain an inert gas, such as nitrogen, argon, or helium, as a carrier gas for carrying alcohol vapor.
The processing chamber need not be separated from the atmosphere, i.e., the processing chamber may be open to the atmosphere, because the ashing process need not be carried out in a vacuum atmosphere. Accordingly, the processing vessel defining the processing chamber need not be airtight. An ashing apparatus having such a processing vessel has a simple construction, which may be known.
A desirable processing temperature is in the range of 150.degree. C. to 250.degree. C.
An organic film removing apparatus of the present invention to achieve the second object comprises: